Strelitzia is actually Experiment X and it's not hard to see why
Since Skuld being Experiment X is almost disproved because of the time stamps relating to Missing Link... I have another theory
Who X truly is that nobody else have ever thought of—
It's Strelitzia
But first I'm going to discuss why Skuld theory no longer works
Spoilers for Melody of Memory, Union X, and Luxu's identity
There is a theory that Skuld is X, the person that was founded by Ansem and experimented on. X is also what drove Isa (Saïx) and Lea (Axel) to do what they did. They often talked to her and planned to help her escape but before they could she was moved. they planned to infiltrate the organization to save her, however they soon lost sight of what their goal was.
In Missing Link it is noted it is place full of Ephemer's descendants. However, in Dark Road, Xehanort has a "dream" vision when talking to (Player) of the adventures Player had and refers that a girl with dark hair (Skuld) looking like his mom.
It is noted that X had amnesia and it seems many people after going through the time travel pods loose memories. Kairi who was experimented and forced to travel to destiny Islands forgot her memories as well. Ven forgot his memories (though it could be from abuse from Xehanort and being separated from Vanitas). But Player also loses their memories in KHML
Which is why it makes the most sense that Strelitzia is X.
It's probably implied they get together. Now his mother is implied to be Skuld meaning they had kids and then she has Xehanort awhile afterwards. This means after their time something happens for her and the others to come back. It wouldn't make sense for her to be X and then go back to Ephemer.
But what if he also took her to Ansem and he did experiments on her. After all, Luxu helped showed where Terra—Xehanort was so how come he couldn't do the same for Experiment X. Ansem then did experiments on her. After that Luxu eventually took her and brought her to Quadratum where she stays.
Strelitzia dies but after the dandelions go off, many of the characters see Strelitzia. It is even shown that Luxu or maybe even MOM might have taken Strelitzia somewhere else such as the Quadratum... Where we do end up seeing her in the KH4 trailer
It adds up the most and there's little reason why it can't work.
